This invention relates to a printing positioning mechanism for adjusting the positional relationship between a print head and a platen when the print head is moved toward the platen and a printer in which a printing positioning mechanism is used.
Conventionally, in a printer such as, for example, a thermal printer, in order to obtain a high-quality print image, a print head such as thermal head is contacted with a platen under a prescribed pressure. The pressure contact of the thermal head is generally performed such that the thermal head which is mounted for movement into and out of contact with the platen (head down and head up) is uniformly pressed against the platen using a plurality of springs so that printing and transportation can be performed in stability. Therefore, it is a first requirement for maintaining high print quality of the thermal printer to set the thermal head in parallel to the platen.
However, since an installation state of the thermal printer, distortion, warping or the like of the thermal head caused by aged deterioration and so forth have an influence on the platen, pressing force of the thermal head is dispersed among different positions of the platen, resulting in gradual appearance of irregularity of the density or blurring. As a result, the print quality of the thermal head is degraded.
Thus, in order to correct such dispersion of the pressing force of the thermal head as just described, various techniques have been proposed wherein an aligning mechanism is applied to the thermal head so that the thermal head can move following up a fixed position of the platen to maintain a parallel state between the platen and the thermal head.
An apparatus for such aligning of a thermal head as described is disclosed, for example, in Japanese Patent Laid-Open No. Hei 2-113955. In the apparatus, a head supporting member for supporting both sides of a thermal head is formed as a channel-shaped plate, and fitting holes for a support shaft for supporting the thermal head for pivotal motion into or out of contact with a platen through the head supporting member are formed as elongated holes at base portions on the opposite upright sides of the channel-shaped plate. The fitting holes are individually formed so as to extend in a pressing direction of the thermal head against the platen such that the head supporting member can freely move toward the platen. Consequently, automatic aligning of the thermal head can be easily achieved.
Meanwhile, a three-dimensional aligning mechanism is disclosed in Japanese Patent Laid-Open No. 2002-234204. In the three-dimensional aligning mechanism, a fulcrum bearing is set as a center of movement of a thermal head with respect to a support shaft for the thermal head such that the thermal head can be moved not only in a two-dimensional directions including an axial direction of a platen and a direction of movement toward or away from the platen but also in a perpendicular direction to the two-dimensional directions.
However, in both of the apparatus disclosed in Japanese Patent Laid-Open No. Hei 2-113955 and No. 2002-234204, an aligning mechanism is applied to a thermal head, and the thermal head apparently falls into a free state as the aligning mechanism of the thermal head with respect to the platen is provided in an increasing number of directions. Therefore, there is a subject to be solved that accurate setting of the thermal head before a printing operation is performed and fixation and maintenance of the thermal head during printing operation become rather difficult.
Further, even if the accurate setting and so forth of the thermal head can be implemented, since a burden is applied to an assembly of the thermal head for which the precision is required, the failure rate of the thermal head increases and the cost of the thermal printer increases from the complexity of the mechanism of the assembly.
In this manner, both of the prior art apparatus have problems in regard to the reliability and the cost although an aligning mechanism is applied to a thermal head to uniformize the pressing force to a platen to enhance the printing quality.